Method of making an electronic device having a thin film resistor formed on an LCP solder mask and related devices

ABSTRACT

A method of making an electronic device may include forming at least one circuit layer that includes solder pads on a substrate and forming at least one liquid crystal polymer (LCP) solder mask having mask openings therein. The method may also include forming at least one thin film resistor on the LCP solder mask and coupling the at least one LCP solder mask to the substrate so that the at least one thin film resistor is coupled to the at least one circuit layer and so that the solder pads are aligned with the mask openings.

FIELD OF THE INVENTION

The present invention relates to the field of electronic devicesfabrication, and more particularly, to a method of making an electronicdevice that includes a liquid crystal polymer solder mask and relateddevices.

BACKGROUND

It may be particularly desirable to reduce the size of a mobile wirelesscommunications device, for example, to a thinner form factor. Reducingthe sire of a device generally requires thinner substrates andcomponents coupled to the substrates. Additionally, it may also bedesirable to reduce fabrication costs, for example, by providing asimpler fabrication process. For example, one approach to reducingsubstrate fabrication costs may include the use of a double-sidedflexible substrate and a solder mask or coverlay, as a less expensiveoption compared to a multilayer flexible substrate and a coverlay.

Additionally, increased use of relatively high frequency radar devicesin the automotive industry (e.g., parking sensors, backup sensors, blindspot sensors, etc.) coupled with the desire for non-hermetic packagingdriven by high volume cost concerns may also increase the desire forlightweight, low cost, high frequency capable packaging and substratematerials. Liquid crystal polymer (LCP) may be considered one suchmaterial, for example.

The use of thinner components may generally require the use of a higherpercentage of bare die and/or flip chip components, for example.Accordingly, mounting those components to a substrate without increasingfabrication costs may be increasingly a concern. Ultrathin flexiblecircuits with reduced thickness active and passive components may thusbe used to address this concern. Thin film passive devices, for examplethin film resistors, have generally been available for several decades.However, a thin film passive device typically must he embedded within amultilayer printed circuit board (PCB), which may not be possible, forexample, with certain flexible circuits.

Existing coverlay materials for flexible circuits generally require anadhesive-based bonding that includes relatively large apertures to beopened around pads for subsequent electrical interconnection between theflexible circuit and the electronic assembly. These coverlay materialshence generally allow for the addition of little if any additionalfunctionality outside of protection of the underlying features on theflex circuit. Accordingly, additional fabrication methods to includeadditional device functionality are needed.

SUMMARY

A method of making an electronic device may include forming at least onecircuit layer that may include a plurality of solder pads on a substrateand forming at least one liquid crystal polymer (LCP) solder mask havinga plurality of mask openings therein. The method may also includeforming at least one thin film resistor on the LCP solder mask andcoupling the at least one LCP solder mask to the substrate so that theat least one thin film resistor is coupled to the at least one circuitlayer and so that the plurality of solder pads is aligned with theplurality of mask openings. Accordingly, the thickness of the electronicdevice may be reduced while maintaining functionality and robustness.

The at least one LCP solder mask may be coupled to the substrate so thatthe at least one thin film resistor is sandwiched between the at leastone LCP solder mask and the substrate, for example. Forming the at leaseone circuit layer may include forming a plurality of circuit layerscarried by opposing surfaces of the substrate, and forming the at leastone LCP solder mask may include forming a plurality of LCP solder masks.Forming the at least one thin film resistor may include forming at leastone thin film resistor on each of the plurality of LCP solder masks, andeach of the plurality of LCP solder masks may be coupled to the opposingsurfaces of the substrate, for example. Each LCP solder mask may becoupled to a respective one of the surfaces of the substrate so that theplurality of thin film resistors are sandwiched between the plurality ofLCP solder masks and the substrate.

The at least one thin film resistor may be formed on the LCP solder maskby sputtering, for example. The method may further include attaching acircuit component to corresponding solder pads of the plurality thereof.

The method may further include forming at least one electricallyconductive via through the substrate. The substrate may include an LCPsubstrate, for example. The substrate may include a flexible printedcircuit board (PCB).

A device aspect is directed to an electronic device that may include asubstrate and at least one circuit layer that includes a plurality ofsolder pads carried by the substrate. The electronic device alsoincludes at least one liquid crystal polymer (LCP) solder mast having aplurality of mask openings therein and at least one thin film resistorcarried by the LCP solder mask. The at least one LCP solder mask may bedirectly coupled to the substrate without an adhesive layer so that theat least one thru film resistor is directly coupled to the at least onecircuit layer and so that the plurality of solder pads is aligned withthe plurality of mask openings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view schematic diagram of part of an electronicdevice in accordance with the prior art.

FIG. 2 is an assembled schematic diagram of a part of the electronicdevice of FIG. 1.

FIG. 3 is an exploded view schematic diagram of part of an electronicdevice in accordance with the present invention.

FIG. 4 is an assembled schematic diagram of a part of the electronicdevice of FIG. 3.

FIG. 5 is a flowchart of a method of making the electronic device inFIG. 4.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring initially to FIGS. 1 and 2, an electronic device 120 madeaccording to a prior art approach is now described. Circuit layers 121that each include solder pads 122 are carried by a substrate 123.Electrically conductive vias 132 expend through the substrate 123 andcouple solder pads 122 on opposing surfaces of the substrate. Anadhesive layer 141 is carried by the substrate 123 and has openings 142therein. A solder mask 124 or coverlay that has mask openings 125therein is carried by the adhesive layer 141. The mask openings 125 arealigned with the adhesive layer openings 142.

The circuit layers 121 each include a thin film resistor 126 carried bythe substrate 123 coupled to corresponding solder pads 122. Each thinfilm resistor 126 is typically formed on a copper substrate, forexample, and coupled within the corresponding circuit layer 121,including the copper substrate, during multilayer lamination. Because itis highly desirable that each thin film resistor 126 be protected orembedded, another adhesive layer, for example in the form of a bondingfilm layer 143, is formed or coupled over each thin film resistor. Inother words, it is highly desirable that each thin film resistor 126 beembedded. In this case, the bonding film layer 143 is sandwiched betweenadjacent substrates 123, or more particularly, adjacent circuit layers121. A circuit component 133 is coupled to corresponding solder pads122, for example, using solder 134.

A disadvantage of the prior art approach described with respect to FIGS.1 and 2 is that to achieve the desired protection or embedding of thethin film resistors 126, multiple layers are used, for example, theadhesive and bonding film layers 141, 143 and two substrates 123. Thus,these layers increase the overall size of the electronic device 120.

Referring initially to FIGS. 3 and 4, and the flowchart 50 in FIG. 5,beginning at Block 52, a method of making an electronic device 20 isdescribed. The method includes, at Block 54, forming electricallyconductive vias 32 through a substrate 23. At Block 56, the method alsoincludes forming a pair of circuit layers 21 a, 21 b that each includessolder pads 22 a, 22 b on opposing surfaces of the substrate 23. Thesubstrate 23 may be a liquid crystal polymer (LCP) substrate, forexample. The substrate 23 may alternatively or additionally be aflexible printed circuit board (PCB). Of course, the substrate 23 may beanother type of substrate, for example.

The method also includes forming a pair of LCP solder masks 24 a, 24 bor coverlays each having mask openings 25 a, 25 b or apertures therein(Block 58). The LCP solder masks 24 a, 24 b may be formed according toany of a number of techniques as will be appreciated by those skilled inthe art. Moreover, while a pair of LCP solder masks 24 a, 24 b isillustrated, it will be appreciated that any number of LCP solder masksmay be used.

At Block 60, the method also includes forming a thin film resistor 26 a,26 b on each of the LCP solder masks 24 a, 24 b. Each thin film resistor25 a, 26 b may include nickel and chromium, e.g., nichrome, amongstother materials (e.g., SiCr, TaN, NiCrAlSi, etc.). It should beappreciated that while two thin film resistors 26 a, 26 b areillustrated, any number of thin film resistors may be formed on each ofthe LCP solder masks 24 a, 24 b. Each thin film resistor 26 a, 26 b maybe formed on the respective LCP solder mask 24 a, 24 b by sputtering,for example. Of course, each thin film resistor 26 a, 26 b may be formedon the LCP solder mask 24 a, 24 b by different or other techniques aswill be appreciated by those skilled in the art.

The method further includes, at Block 62, coupling each of the pair ofLCP solder masks 24 a, 24 b to the opposing surfaces the substrate 23 sothat each thin film resistor 26 a, 26 b is coupled to a respective oneof the pair of circuit layers 21 a, 21 b and so that each of the solderpads 22 a, 22 b is aligned with respective mask openings 25 a, 25 b.More particularly, each LCP solder mask 24 a, 24 b is coupled to arespective one of the surfaces of the substrate 23 so that the thin filmresistors 26 a, 26 b are sandwiched between the LCP solder masks and thesubstrate. In other words, the LCP solder masks 24 a, 24 b are directlycoupled to the substrate 23 without an intervening adhesive layer.

At Block 64, the method also includes attaching a circuit component 33to corresponding solder pads 22 a. The circuit component may be anintegrated circuit (IC), for example. The circuit component 33 may beanother type of circuit component as will be appreciated by thoseskilled in the art. The circuit component may be coupled to therespective solder pads 22 a with solder 34, for example, solder balls.The method ends at Block 66.

Indeed, relative to the prior art electronic device 120, the thicknessof the electronic device 20 is significantly reduced in that theelectronic device includes one, instead of two, substrates 23 andcircuit layers 21, does not include a bonding film layer 143, and doesnot include an adhesive layer 141. Thus, the forming of the thin filmresistors 26 on the LCP solder mask 24 may be considered to make the LCPsolder mask 24 functional as the thin film resistor is formed directlyon the LCP solder mask.

A device aspect is directed to an electronic device 20 that includes asubstrate 23 and at least one circuit layer 21 a, 21 b that includes aplurality or solder pads 22 a, 22 b carried oy the substrate. Theelectronic device 20 also includes at least one liquid crystal polymer(LCP) solder mask 24 a, 24 b having a plurality of mask openings 25 a,25 b therein and at least one thin film resistor 26 a, 26 b carried bythe LCP solder mask. The at least one LCP solder mask 24 a, 24 b is alsodirectly coupled to the substrate 23 without an adhesive layer so thatthe at least one thin film resistor 26 a, 26 b is directly coupled tothe at least one circuit layer 21 a, 21 b and so that the plurality ofsolder pads 22 a, 22 b is aligned with the plurality of mask openings 25a, 25 b.

As will be appreciated by those skilled in the art, the electronicdevice 20 made by the method described herein may be particularlyadvantageous for ultrathin electronic substrates, such as those used inmobile phones and tablets where thinning may be highly desirable.Indeed, the method described herein provides an adhesive-less bondingmechanism, which supports the increased functionality of the LCP soldermask 24 a, 24 b or coverlay. For example, the electronic device 20 madeby the method described herein may achieve near hermiticity, relativelyhigh tensile strength and abrasion resistance which may serve to protectthe underlying thin film resistors from damage while providing stabilityof resistor values, for example, commonly in the range of 25 Ohms persquare (OPS) to 150 OPS. Increased density signal routing and 3Dintegration may also result, as well as increased wideband dielectricproperties that generally do not change upon exposure to moisture. Withrespect to the coefficient of thermal expansion (CTE), the electronicdevice made according to the method described herein may be relativelycompatible with the most common integrated circuit die materials, andmay generally be chemically inert and biocompatible within the humanbody, for example.

Further details of an electronic device having an LCP solder mask andrelated methods may be found in U.S. Pat. Nos. 8,693,203 and 8,472,207and U.S. Patent Application Publication No. 2012/0182702, all of whichare assigned to the assignee of the present application, have a commoninventor, and all of which are hereby incorporated by reference in theirentirety.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A method of making an electronic devicecomprising: forming a pair of circuit layers each comprising a pluralityof solder pads on opposing major surfaces of a liquid crystal polymer(LCP) substrate; forming a pair of LCP solder masks each having opposingfirst and second major surfaces and a plurality of mask openingsextending therebetween; forming at least one thin film resistor directlyon the first major surface of each of the pair of the LCP solder masks;and after forming at least one thin film resistor directly on the firstmajor surface of each of the pair of LCP solder masks then coupling eachof the pair of LCP solder masks to the opposing major surfaces of theLCP substrate so that each at least one thin film resistor is coupled toa respective one of the pair of circuit layers and so that eachplurality of solder pads is aligned with a respective plurality of maskopenings.
 2. The method of claim 1 wherein each LCP solder mask iscoupled to a respective one of the major surfaces of the LCP substrateso that the plurality of thin film resistors are sandwiched between theplurality of LCP solder masks and the LCP substrate.
 3. The method ofclaim 1 wherein the plurality of thin film resistors is formed on theplurality of LCP solder masks by sputtering.
 4. The method of claim 1further comprising attaching a circuit component to corresponding solderpads of the plurality thereof.
 5. The method of claim 1 furthercomprising forming at least one electrically conductive via through theLCP substrate.